A good place for practicing reversing with the trailer is at the Silverwater ramp (right under the Silverwater bridge). There's a huge car park there which is rarely used (except in winter where there may be a few soccer players), and there are not many people there to see you make a fool of yourself. A word of caution though, if you intend to launch at the ramp wait untill all river cats are well out of the way as their wash will cause problems.
One simple trick which I have learned when reversing is to hold the steering wheel at the bottom. If you want the trailer to go to the right, move your hand to the right, same thing for the left.
As has been said before, double check the bungs! I also make sure the engine support bracket is off but leave the engine up (don't want to scrape the skeg).
Also, I always pull out the brake light plug from the car before immersing the trailer so the lights aren't operating in the cold water - never blown a bulb in three years.
Happy boating!